Methods of molecular cytogenetics will be used to gain further insight into the organization of specific DNA sequences in heterochromatin. The relative positions of the controlling element, ribosomal RNA cistrons, and satellite DNAs in the fungus gnat Sciara coprophila will be mapped by in situ hybridization to polytene chromosomes of stocks carrying translocations within the X-heterochromatin. Whether or not there are deletions or modifications of satellite DNAs in primary spermatocytes, suggesting possible functional roles for satellite DNAs in chromosome recognition and/or movement, will be determined by Q-banding and in situ hybridization. Fragments of DNA from the Sciara genome will be cloned in order to select DNA from the controlling element and the adjacent centromere, for further molecular characterization by restriction enzyme analysis and DNA sequencing.